Hitherto, a brake device equipped with a booster for lightening a brake pedal depression with the aid of vacuum produced by the internal combustion engine has been widely known.
When, after taking the homogenous combustion by providing engine cylinders with a homogenous air/fuel mixture, the internal combustion engine of the vehicle is switched to take the stratified combustion by feeding the surrounding of ignition plugs with a richer air/fuel mixture, it becomes necessary to largely open the throttle valve for taking much larger air than that in case of the homogeneous combustion, so that under the stratified combustion, the vacuum produced by the internal combustion engine is lowered (viz., approaching the atmospheric pressure).
For example, in Patent Document 1, there is disclosed a technology in which when, in case of obtaining a vacuum for use as a power source for the booster, a desired value of a braking force is equal to or lower than a predetermined value, switching from the stratified combustion to the homogenous combustion is inhibited.
The technology of Patent Document 1 is based on the premise that the magnitude of the braking force is determined in accordance with a vehicle speed, and thus, when the vehicle speed is high, judgment is so made as to need a higher braking force. When the vehicle is running on an upslope, reduction of the vehicle speed inevitably takes place, and thus, as the gradient of the upslope increases, judgment is so made that a larger braking force is not needed. In Patent Document 1, there is also disclosed such a technology that a judgment level of the braking force may be varied in accordance with the inclination of a road surface.
However, in the technology disclosed by Patent Document 1, even when, with the vehicle being at a standstill on an inclined road, the judging level of the braking force is controlled to vary in accordance with the inclination of the road surface, the magnitude of the braking force is forced to be judged based on the vehicle speed, and thus, there is the possibility that a desired braking force is not obtained at a standstill of the vehicle. In other words, when, in the technology of Patent Document 1, a vehicle is at a standstill on an inclined road, there is such a possibility that the switching between the stratified combustion and the homogenous combustion is not appropriately made in the engine.
In view of the above, in the present invention, when a vehicle is at a standstill after cold starting and thus the catalyst in an exhaust gas purifying device installed in an exhaust passage of the engine is not sufficiently activated, a control is so made that an operation range of the stratified combustion at a standstill after cold starting is increased in accordance with an inclination (viz., inclination angle) of the vehicle to suppress an unnecessarily frequent homogenous combustion for the purpose of obtaining a vacuum led to a brake booster to lighten the brake pedal depression.